Power for Generations Siemens Power Generation

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BENSON Boilers for MaximumCost-Effectiveness in Power Plants

Implementation of drum boilers is restricted to subcriticalpressures.The BENSON boiler enables realization of bothsubcritical and supercritical pressures.

Operating pressure

Water wall tubing

Principle Once-through(BENSON)

20 ... 400 bar

spiral or vertical

Economizer

Evaporator

Superheater

Natural circulation(Drum)

10 ... 180 bar

vertical

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BENSON Boilers – Proven a Thousand Times Over

With over 1000 units built,the BENSON boiler is by farthe most widespread once-through boiler.

Principle:

• Water is preheated and evap-orated and steam is subse-quently superheated in a singlepass.

• A feedwater pump generatesforced flow in the evaporator.

Most important characteristics:

• Depending on operatingconditions, the evaporationendpoint shifts automaticallywithin one or more heatingsurfaces.

• The system can be operatedat subcritical or supercriticalpressures.

Siemens and BENSON boilers

In 1922, Mark Benson sub-mitted a patent applicationfor ”Generating Steam withany Pressure“ – marking thebirth of the once-throughboiler. Based on the conceptdeveloped by Mark Benson,Siemens continued to developthis type of boiler in the mid-twenties while retaining thename BENSON boiler. It wasrecognized that a once-throughevaporator could be operatedat both subcritical and super-critical pressure.

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BENSON Boilers for Low Life-Cycle Costs

Cost-effectiveness in high-output steam power plants isdetermined by investmentcosts, process and componentefficiencies, availability, andmaintenance costs.The boilerplays a leading role due to itskey function in the water/steamcycle and its large contributionto investment costs.

Overview of the advantagesof BENSON boilers:

• Highest achievable plantefficiency with supercriticalsteam parameters

• High plant efficiency evenat part load

• Suitable for all coal gradesavailable on the world market

• Short startup times

• Sliding-pressure operationwith high load transients

• Proven thousands of timesover and available worldwide

• Continuous further devel-opment of this technology.

The transition from subcritical tosupercritical steam pressuresignificantly increases processefficiency, with a considerabledecrease in fuel costs. Investi-gations such as those of theCoal Industry Advisory Board ofthe International Energy Agencyhave shown that this reduceslife-cycle costs:

The effect of low fuel consump-tion on electric power generationcosts is greater than the onlyslightly higher investment costsfor the power plant.

Franken II power plant, Germany2 4 200 MWSupercritical pressure, double reheatBituminous coalBoiler manufacturer: Deutsche Babcock

BENSON technology is available everywhere,as leading boiler manufacturers in all parts of theworld have obtained licenses from Siemens.

Austria

Denmark

Germany

Great Britain

Japan

USA

Italy

India

Austrian Energy &Environment(SGP/Waagner-Biró)

FLS miljø(Burmeister & Wain Energi)

Babcock Borsig Power(Deutsche Babcock,Steinmüller)

Mitsui Babcock Energy

Babcock-HitachiKawasaki Heavy Industries

Babcock & WilcoxFoster Wheeler

Ansaldo

Bharat HeavyElectricals Ltd. (BHEL)

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Licensees

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Power plants are currentlybuilt for a service life of 30 to40 years.They must thereforecontinue cost-effective electricpower generation through theyear 2040. High efficiencycontributes significantly tothe cost-effectiveness ofsteam power plants.

One of the most effective mea-sures for achieving high powerplant efficiency is selecting ahigh design steam pressure.Efficiency increases by roughly3% on making the transitionfrom 167 bar (e. g. drum boiler)to 250 bar, without significantincreases in investment costs.The increased costs for thegreater wall thicknesses of thepressure section are largelycompensated by the lower costsof the smaller fuel/air/flue-gaspath.

The BENSON boiler can be builtwith essentially the same designfor subcritical and supercriticalpressures. Only the dimensionsand wall thicknesses of the tubesand headers change with increas-ing pressure, or the materialslimits are shifted.

Main steam properties areselected based only on aspectsof cost-effectiveness.

BENSON boilers have been builtto date for pressures from 40 to310 bar.

Highest Efficiency with Supercritical Pressure

Increasing steam pressure is the most effective way to increasethe efficiency and hence the cost-effectiveness of a power plant.

A BENSON boiler which is designed for supercritical pressurescan be operated at any pressure, i. e. also at subcritical pressure.

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High Main Steam Temperatures Even at Part Load

The variable evaporation endpoint in BENSON boilers enablesachievement of high main steam temperatures over a large outputrange independent of operating conditions.

Lippendorf power plant, Germany2 4 936 MWSteam conditions:260 bar/554iC/583iCLigniteBoiler manufacturer: Deutsche Babcock

Main steam temperatures inthe BENSON boiler are inde-pendent of load.This resultsin a higher process efficiencyfor the power plant over awide load range.

The fuel/feedwater flow ratio iscontrolled in the BENSON boilersuch that the desired steamtemperature is always establishedat the main steam outlet. This ismade possible by the variableevaporation endpoint. The eva-poration and superheating surfa-ces automatically adjust to ope-rating conditions. In dynamic pro-cesses, desuperheaters supportmaintenance of constant mainsteam temperature.

Minimum output in once-throughoperation at high main steamtemperatures is 35 to 40% forfurnace walls with smooth tubesand is as low as 20% if rifledtubes are used.

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The size and geometry of thefurnace of a BENSON boilercan be optimally matched tothe fuel with no restrictionson the water/steam side.

The BENSON boiler enablesdimensioning of the furnacebased solely on combustionengineering aspects:

• The transition from evaporationto superheating is not fixed inlocation and can take place atany point in the upper sectionof the furnace. This enables

BENSON Boiler – Suitable for Every Fuel

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Tangential Slag tapFour wallCornerOpposedFront

Lignite 930 MW

Anthracite 350 MW

Anthracite 770 MW

Bit.coal 400 MW

Bit.coal 400 MW

Bit.coal1400 MW

Bit.coal 320 MW

Down shot

BENSON licensees have extensive experience with the fullrange of firing systems. Vortex burners are the most prevalent;jet burners are also implemented in tangential firinggeometries.

Meri-Pori power plant, Finland560 MWSupercritical pressureBoiler manufacturer: Mitsui Babcock Energy/Tampella

dimensioning of the furnacewithout restrictions on thewater/steam side.

• Forced flow in the evaporatortubes also enables implemen-tation of complex furnacegeometries such as for slagtap firing.

The BENSON boiler is constructedwith the widest possible rangeof firing systems. Plants are alsoconstructed with fluidized-bedcombustion systems in additionto the examples shown.

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Single-pass• Simple construction of membrane

walls• Low erosion of heating

surfaces by ash• Small building area

Two-pass• Dimensioning of convection section

independent of furnace• Shorter assembly times due to parallel

assembly of first and second pass• Low building height

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BENSON boilers are suitable for all fossil fuels as well as for nuclear power plants.More than 1,000 BENSON boilers have been built to date.Their cumulative steamoutput of 200,000 kg/s corresponds to a power plant output of roughly 220,000 MW.

The BENSON boiler design isadapted to the fuel and localconditions.

The following concepts haveproven themselves for the variousfuels:

• Bituminous coal:Opposed firing, single-pass ortwo-pass design

• Lignite:Tangential firing, single-passdesign

• Anthracite:Arch firing (dry or wet ashremoval) or opposed firing,single-pass or two-passdesign

Even if the coal grade varieswithin a wide range, steamoutput and main steam tem-perature are always achievedin the BENSON boiler.

The two main configurations for the BENSON boiler to date are the single-pass andtwo-pass design

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

200,000

1946 1956 1966 1976 1986 1996

kg/s

Cumulative steam capacity of ordered BENSON boilers

1998

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Bituminous coal, dry de-ashing

Bituminous coal, slag tap firing

Lignite

Oil or gas

Combined cycle

Nuclear

Thermoelastic Construction for Flexible Operation

The allowable temperature transientsfor the separator of a 250-bar BENSONboiler are roughly three times as largeas for the drum in a 175-bar drumboiler.

Instead of a thick-walled drum, theBENSON boiler has several thin-walledseparators configured for parallel flow.

Majuba power plant, South Africa3 4 660 MW + 3 4 715 MWHigh-ash coal Boiler manufacturer: Steinmüller

The thermoelastic construc-tion and small thermal storagemasses in the BENSON boilerenable flexible power plantoperation with short startuptimes and large load transientsover a wide output range.

The BENSON boiler is character-ized by thermoelastic con-struction which results in onlylow thermal stresses due totemperature changes. There isno thick-walled drum at the endof the evaporator; instead thereare only several thin-walledseparators with high allowabletemperature transients.

A further advantageous charac-teristic of the BENSON boilerare the small thermal storagemasses, which also have apositive effect on flexibility.

Short startup times and highoutput transients over wide out-put ranges are therefore possiblefor the BENSON boiler.

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Optimized control conceptsand the advantages of theBENSON principle result inexceptional operating behavior.

Control of a BENSON boilerdiffers from that of a drum boilerprimarily only in the controlparameter for feedwater control.

The setpoint for this controlparameter is automaticallyacquired in the BENSON boilerfrom a shift in heat transferbetween the evaporator andsuperheater. This accounts forload dependency, the degree offouling of the heating surfacesand conditions such as HP feed-water heater out of service. Thesetpoint is selected such thatthe main steam temperaturealways remains constant underall operating conditions.

Control – Proven and Reliable

The proven control principles for steam boilers also hold for BENSON boilers.Measured values on the water/steam side influence feedwater control, while thosefrom generator output and steam pressure are used to generate the setpoint forfuel control.

Drum water level is used for controlling the feedwater flow in a drum boiler, whiletemperature or enthalpy downstream of the evaporator are used in a BENSON boiler.

∆�set

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pressure set

Pelset Pelactual

hset

Enthalpycomputer

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Hekinan power plant,Japan

639 kg/s - imported coal255 bar/543 IC/569 ICManufacturer: Babcock-Hitachi

Hemweg power plant,Netherlands

530 kg/s - imported coal261 bar/540 IC/540 ICManufacturer: Mitsui Babcock Energy/Stork

Nordjyllandsvaerketpower plant, Denmark

270 kg/s - imported coal310 bar/582 IC/580 ICManufacturer: Burmeister & Wain

Completed BENSON Boilers

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Aghios Dimitrios power plant,Greece

286 kg/s - lignite 242 bar/540 IC/540 ICManufacturer: AE&E

Ibbenbüren power plant,Germany

600 kg/s - anthracite220 bar/530 IC/530 ICManufacturer: Steinmüller

Lippendorf power plant,Germany

672 kg/s - lignite285 bar/554 IC/583 ICManufacturer: Deutsche Babcock

Furnace (under construction) in a 936-MW unit at Lippendorf power plant

Startup – Low Losses and Low Thermal Stresses

The BENSON boiler startup system consists of several separators, a collectingvessel and a flash tank. Installation of a circulation pump is cost-effective forfrequent startup, as this further reduces startup losses.

Mehrum power plant, Germany660 MWDaily startup and shutdownBoiler manufacturer: Deutsche Babcock/Steinmüller

BENSON boilers with provenstartup and bypass systemsyield short startup times whileimposing the lowest possiblestress on materials.

BENSON boilers can be startedup easily and rapidly, and withlow life-limiting effects on thematerial, using the same method,from any operating condition.The integrated startup system isin operation only during startup.

The unit is normally equippedwith a turbine bypass system.The high-pressure turbine as wellas the intermediate-pressure/low-pressure turbines are equippedwith separate bypass valves andlines. This enables flow throughall superheater surfaces and thusreliable cooling even shortly afterignition.

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A coal-fired unit with aBENSON boiler can sustainload transients of 4 to 6%/mineven over a wide outputrange.

Temperature changes occur inthe boiler and in the turbineduring load changes. These cancause thermal stresses in thick-walled components. These areespecially high in the turbineduring constant-pressure opera-tion. They therefore limit themaximum load transient for theunit. By contrast, in sliding pres-sure operation, the temperaturechanges are in the evaporatorsection. However, the resultingthermal stresses are not limitingin the BENSON boiler due to itsthermoelastic design.

Power plants with BENSONboilers are therefore especiallywell-suited to sliding-pressureoperation. This operating modeis required by most customersworldwide.

As the adjacent figure shows,the advantage of the variableevaporation endpoint becomesespecially clear in sliding-pressureoperation.

The enthalpy increase in theboiler for preheating, evaporationand superheating changes withpressure. However, pressure isproportional to output in sliding-pressure operation. In a uniformlyheated tube, the transitionsfrom preheat to evaporation andfrom evaporation to superheatshift automatically with load suchthat the main steam temperaturealways remains constant.

Flexible Sliding-Pressure Operation

In fixed-pressure operation, temperatures change in the turbine when load changes,while in sliding-pressure operation, they change in the boiler.

The fractions for preheat, evaporation and superheat change with pressure in the boiler(left). In sliding-pressure operation, the size of the heating surfaces automaticallyadapts to these conditions (right).

Pressure Load

Evaporation

Superheating

Preheating

400

Enthalpy increase along the tube length

20

40

60

80

100

60 80 100

Enthalpy

Boiler outlet

Criticalpressure

Boiler inlet

%

%50 100 150 200 250 bar 350

Load

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Customers require the highestpossible reliability for theirplants based on scientificallyfounded design fundamentals.

The excellent operating experi-ence and high reliability of theBENSON boiler are based on

• intensive research anddevelopment at Siemens.The BENSON test rig plays a central role in this.

• continuous exchange of infor-mation between Siemens andBENSON licensees and cus-tomers as well.

Important tasks for the BENSONtest rig are measurement ofheat transfer and pressure dropin tubes as well as investigationsof thermoelastic componentsand feedwater considerations.

The test matrix gives an over-view of the measurementswhich have been systematicallyperformed since 1975 for awide range of parameters.

These measurements enabledgeneration of scientifically found-ed design documents as well assuch achievements as increas-ing heat transfer in rifled tubesby approximately 50% throughoptimization of rifling geometry.

Siemens R&D Ensures the Technological Advantage of the BENSON Boiler

The BENSON test rig in Erlangen is one of the highest-performance test facilitiesfor the water/steam cycle anywhere in the world. It is constantly being adapted tocurrent requirements.

uniform uniformone side one side

> 100.000 > 160.000

Heating

Number ofmeasurements (1998)

Tubeorientation

vertical

inclined

horizontal

Smooth RifledTube geometry

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2 4 1100-MW power plant, USAVertically-tubed furnace (UP design)Boiler manufacturer: Babcock & Wilcox

Investigations over several decades have enabled Siemens to establish the world'slargest database for heat transfer and pressure drop in tubes.

The outstanding heat transfercharacteristics of rifled tubesenable simplification of thedesign of the BENSON boilerand further improvement inits operating behavior.

The rifled tube has proven itselfin operation in several hundredboilers since the '70s. Siemensuses this experience to signifi-cantly improve rifling geometrywith regard to heat transfer.

The upper part of the figureshows tube wall temperaturesat the same mass flux for asmooth tube, a standard com-mercial rifled tube and a rifledtube optimized by Siemens. Thelower portion of the figuredemonstrates how it is possibleto reduce mass flux in an opti-mized rifled tube by roughly halfthat in a smooth tube at thesame wall temperature. Thisresult has led to an innovativefurther development of the ver-tically tubed BENSON boiler.

Rifled Tubes – Design Elements of the Future

1800 2000

2200 2400

Pressure 212 barPeak heat flux 310 kW/m2

2200 2400

2200 2400

1800360

380

400

440

2000 2200 2400

Fluid enthalpy

Inner wall temperature

360

380

400

°C

440

Smooth tubeMass flux 1000 kg/m2s

Standard rifled tubeMass flux 1000 kg/m2s

Optimized rifled tubeMass flux 1000 kg/m2s

Smooth tubeMass flux 1500 kg/m2s

Standard rifled tubeMass flux 1000 kg/m2s

Optimized rifled tubeMass flux 770 kg/m2s

kJ/kg

°C

kJ/kg

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The outstanding heat transfer characteristics of the optimized rifled tubecan be utilized to reduce either tube wall temperatures or mass fluxes inrifled tubes.

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The vertically tubed BENSONboiler is a reliable design forthe future due to its simpledesign and optimal operatingcharacteristics. Further devel-opment of the verticallytubed BENSON boiler hasresulted in the generation ofa boiler design combining lowinvestment costs, outstandingoperating characteristics andhigh reliability. Its range ofapplication extends from 300to over 1000 MW.

Of the more than 1000 BENSONboilers constructed to date,roughly 600 have a verticallytubed furnace, most of thesewith several trains connected inseries.

Implementation of optimizedrifled tubes with enhanced heattransfer characteristics now en-able parallel configuration of allfurnace tubes even in verticallytubed BENSON boilers.

BENSON Boiler with Vertically Tubed Furnace

The BENSON boiler with vertical rifled tubes combines the advantages of the BENSONboiler with spiral-wound furnace tubes with the simple design of the drum boiler.

Matsuura power plant, Japan2 4 1000 MW UP boiler with vertical tubes andboiler with spiral-tube furnaceBoiler manufacturer: Babcock-Hitachi

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In the high mass flux design, mass flow through a tube with higherheat input decreases, while in the low mass flux design it increases,thus reducing possible temperature differences at the evaporator outlet.

The special feature of this furtherdevelopment of the BENSONboiler is reduced mass flux inthe evaporator tubes. This leadsto the following behavior of asingle tube with higher heatinput:

• The geodetic pressure dropdecreases due to increasedsteam generation, andthroughput increases.

• Simultaneously, the pressuredrop due to friction increasesas a result of the highersteam quality and the highermass flow.

• However, as the influence ofthe pressure drop due to fric-tion on the mass flow is onlyslight, the mass flow throughthe tube with higher heatinput increases.

This favorable behavior of drumboilers is known and is referredto as the natural circulation char-acteristic. It results in improvedcooling of the tube with higherheat input. Installation of flowrestrictors for more uniform flowdistribution is not required.

BENSON boiler license

Research & development Power plant departments

Boiler know-how Power plant know-how

• Boiler and firing concept• Thermo- and fluid dynamics• Basic design• Simulation of dynamics• Startup system• I&C concept• Water chemistry

• Incorporation of boiler into the power plant• Interaction of boiler, turbine and control• I&C (manufacturing by Siemens)

• Turbine• Piping• Construction• I&C

• Test rig (330 bar/660°C)• Heat transfer, flow phenomena• Thermoelastic components• Feedwater chemistry• Computer programs

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Siemens is the licensor forBENSON boilers. Leadingboiler manufacturers aroundthe world make use of thisconcentrated expertise.

This know-how is continuouslyextended and updated with con-tinuous development and globalexchange of information. Inaddition to BENSON technology,it also includes interaction of theBENSON boiler with the turbineand with other major componentsas the basis for cost-effectiveand reliable power plant operation.

BENSON Technology – Best Choice for Modern Power Plants

Siemens milestones in the field of coal-fired power plants:

1882 Separation of radiant andconvective heat transferareas in boiler first proposedby F. Siemens

1924 Siemens obtains patentrights from Mark Benson anddevelops the BENSON boiler.

1926 Siemens manufactures thefirst BENSON boiler with 30 t/h

1933 Siemens ceases manufac-turing boilers and awardsBENSON licenses to severalboiler manufactures

1933 Siemens introduces variable-pressure operation

1949 The world's first high-pres-sure/high-temperature powerplant with once-through boiler(175 bar/610 iC, BENSONboiler at Leverkusen, Germany)

1954 The world's first steam tur-bine with supercritical steamconditions: 300 bar/600 iC

1962 First directly water-cooledgenerator stator and rotor

1963 The world's first boiler withspiral-wound tubes in mem-brane wall design (BENSONboiler at Freiburg, Germany)

1987 The world's largest coal-firedunit with full arc admissionsteam turbine, boiler withspiral-wound water walls andsliding pressure operation(900 MW unit with BENSONboiler at Heyden 4)

1994 Siemens proposes verticallytubed water walls in low massflux design for BENSON boilers

1995 Order for the world's largeststeam turbine-generator intandem design. Rating:1000 MW (251 bar/575 iC/599 iC for Niederaußempower plant)

2000 1000 BENSON boilers soldwith >700.000 t/h in total

To date over 1000 BENSON boil-ers have been built worldwide.This constitutes roughly 60% ofall once-through boilers (notincluding the states of the formerSoviet Union).

The following operating param-eters have been achieved:

• Output up to 1232 kg/s(4435 t/h)

• Pressure up to 310 bar

• Temperature up to 650°C

Siemens has comprehensive know-how for engineering turnkey power plants.Thisknow-how is the basis for the sucess of the BENSON boiler – a key component ofthe plant.

Siemens Power Generationplans and designs powerplants, constructs turnkeyplants as a general contractor,participates in erection con-sortia or works together witharchitect engineers.

Siemens provides the followingservices for fossil-fueled powerplants:

• Steam turbine power plantswith coal, gas or oil firing

• Gas turbine and combined-cycle plants with both gas andsteam turbines (unfired com-bined-cycle, parallel-poweredand GUD units)

• GUD plants with integratedcoal gasification

• Engine-based cogenerationplants.

Component deliveries:

• Steam turbines up to1200 MW

• Gas turbines up to 260 MW

• Turbine-generators

• Plant and turbine-generatorI&C

• Electrical engineering

• Catalytic reactors and cata-lysts for nitrogen oxide control.

Manufacture of power plantcomponents is supplementedby comprehensive expertise inthe field of BENSON boilers.This combination of first-handpower plant knowledge and freechoice of boiler manufacturer isa typical feature of Siemenspower plants and ensures highcost-effectiveness and high reli-ability.

Services

Siemens supports its customersin every phase of a power plantproject and its subsequent ser-vice life by providing all desiredservices:

• Grid extension planning

• Project development (generation of feasibility stud-ies, elaboration of financingconcepts, support in thelicensing phase, formulationof project contracts, coopera-tion in risk management, par-ticipation in the project com-pany with equity capital)

• Plant operation (participation in operationsmanagement, service andrepairs, upgrading and mod-ernization, backfitting andexpansion, personnel training)

Siemens – Competence in Power Plant Engineering

Installed capacity worldwide

Plant type Overall output (MW)

Fossil-fueled steam power plants 328,000

Gas turbines 92,000

Industrial power plants 27,000

Nuclear power 100,000

Total thermal power 547,000

Hydroelectric 95,000

(including Allis Chalmers, Parsons and Westinghouse)

Development - Planning - Manufacturing - Supply - Construction - Service

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Published by and copyright 2001Siemes AGPower GenerationFreyeslebenstraße 191058 Erlangen, Germanywww.pg.siemens.com

For further details and information,please contact:Siemens AGPower GenerationPG W7P.O. Box 32 2091050 Erlangen, GermanyTel. +49 9131 18-62 34Fax +49 9131 18-62 14e-mail: joachim.franke@erl11.siemens.de

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Order No.: A96001-S90-A469-V2-7600Printed in Germany101502M WS 04011.SEK No. 22209